1. Field of the Invention
The present invention relates to a light-emitting diode (hereinafter, simply referred to as an "LED") array and a method for fabricating the same. More specifically, the present invention relates to an LED array usable for printing a date on a negative film in a data back unit including a date indicating system for a camera and a method for fabricating the same.
2. Description of the Related Art
Referring to FIG. 20, a conventional LED array will be described. FIG. 20 is a top plan view showing a conventional LED array usable for printing a date on a negative film in a data back unit including a data indicating system for a camera. As shown in FIG. 20, a conventional LED array includes seven light-emitting chips 100 arranged in a line. As is apparent from FIG. 20, this LED array has a simple structure easily obtained by arranging a plurality of independent light-emitting chips in a line. An electrode is denoted by the reference numeral 101. Wires connected with the electrode 101 are not shown in FIG. 20. The size of each light-emitting chip 100 is approximately 300 .mu.m.times.300 .mu.m, for example. The size of the entire LED array, including a substrate on which the LED array is mounted, is approximately 1 mm.times.4 mm, for example.
Each LED chip for each LED is formed on a single GaP chip, for example. Each LED chip is made of a single GaAs.sub.0.15 P.sub.0.85. The emission spectrum of each LED chip has a peak in the vicinity of 590 nm. In practical use, this LED array for printing a date on a negative film, a driver IC and a focusing lens are mounted on a substrate.
However, since such a conventional LED array to be used for a data back unit for a camera employs a plurality of single GaAs.sub.0.15 P.sub.0.85 LED chips of an indirect transition type formed on a single GaP chip, the luminous efficiency of these LED chips becomes disadvantageously low. This is why, in order to obtain a sufficient amount of output required for printing a date on a negative film, a current of about 20 mA is required for one LED chip. Therefore, a maximum amount of the current required for printing a date on a negative film reaches 140 mA, i.e., 20 mA.times.7 chips. In order to supply such an amount of current, it is indispensable to provide a driver IC for an LED array for practical use, as described above. The provision of the driver IC is an obstacle to realizing the objectives of reducing the size of the LED array, the cost necessary for fabricating the LED array and the consumption power necessary for operating the LED array.
On the other hand, FIG. 21 shows an exemplary semiconductor LED array to be used as an LED printer, as disclosed in Japanese Laid-Open Patent Publication No. 4-100278. The LED array shown in FIG. 21 uses a direct transition type (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P (where 0.ltoreq.x.ltoreq.1 and 0.ltoreq.y.ltoreq.1) layer as an active layer.
As shown in FIG. 21, the semiconductor LED array includes: an n-type InGaAlP cladding layer 202; an InGaAlP active layer 203; a p-type InGaAlP cladding layer 204; and a p-type GaAlAs layer 205. These layers are deposited in this order on a GaAs substrate 201. In FIG. 21, an n-type electrode is denoted by 206; a GaAs contact layer for forming an electrode is denoted by 207; a p-type electrode is denoted by 208; an n-type InGaAlP insulating layer is denoted by 209; and a bonding pad is denoted by 210.
Since this structure uses a direct transition type (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P (where 0.ltoreq.x.ltoreq.1 and 0.ltoreq.y.ltoreq.1) as the active layer 203, a relatively high output can be advantageously obtained at a low current value. Therefore, by employing this structure, the reduction in the amount of the current required for printing a date on a negative film in a data back unit for a camera can be expected.
In addition, since this structure applicable to a data back unit for a camera is a monolithic LED array as shown in FIG. 21, it is possible to reduce the size of the LED array as compared with the LED array obtained by independently arranging a plurality of light-emitting chips (or elements) in a line as shown in FIG. 20. Moreover, this structure also makes it possible to reduce the spot diameter of the light emitted from the LED, so that an optical system such as a lens, which has conventionally been required for focusing a spot, is no longer necessary. As a result, the size of the entire system using a data back unit for a camera can be reduced. As compared with a conventional LED array using indirect transition type LED chips shown in FIG. 20, the LED array shown in FIG. 21 surely enables the reduction in the amount of the current required for printing a date on a negative film. However, in order to practically use this LED array for a camera, the maximum amount of the light which can be emitted from this LED array is not sufficiently large for the value of the current required for the emission. Therefore, an LED array allowing for emitting a sufficient amount of light at a lower current value is expected to be developed.